Q&A: Stanford scientists discuss what ‘molecular fossils’ could tell us about life on other planets
Stanford Associate Professor Paula Welander and her student Marisa Mayer discuss how microscopic traces of early life – called microbial lipid biomarkers – could help demystify the origins of life and life beyond Earth.
Paula Welander may be a microbiologist, but she spends a lot of time studying fossils. Like the bones left behind by dinosaurs, some of Earth’s earliest inhabitants – microbes – also left behind indicators in the rock record.
Welander’s laboratory at Stanford University examines microbial lipid biomarkers, which are fossils of lipid, or fat, molecules that existed within ancient microbes, to look for insight on how the microbes evolved in the harsh and unforgiving conditions of early Earth. Through this research, she hopes to answer some of life’s greatest mysteries, including how life started here and where else it could be in the universe.
Here, Welander, an associate professor of Earth system science in the School of Earth, Energy & Environmental Sciences (Stanford Earth), and her graduate student Marisa Mayer, who will become a postdoctoral researcher with NASA Ames Research Center this fall, talk about these “molecular fossils,” the close relationship between lipids and life, and where a microbiologist goes to gather their samples.
What are microbial lipid biomarkers?
Welander: Lipids are fats that we have in ourselves – every organism alive today makes lipids – and a microbial lipid is a molecule made by a microbe.
A biomarker is an indicator that geologists can use to let them know that life was here – that this was left by a living organism. It could be a fossil bone, for example. But a molecule that’s made today by an organism can also be fossilized and turned into a biomarker. So, a microbial lipid biomarker is a certain type of indicator of microbial life in the rock record.
I can’t help but think of Jurassic Park!
Welander: Yes, exactly! Microbes of Jurassic Park.
Can you give us an idea of what your experiments are like?
Welander: Our experiments can vary depending on the project. A lot of the experiments that I did when I was a student and a postdoc involved growing microbes in the lab and then using bioinformatics and computational approaches to identify genes that might be important in making a microbial biomarker within the organism. And then we test these genes and proteins in the lab and show that they do actually make these biomarker lipids.
Mayer: I am a little unique in the Welander lab in that I do fieldwork. I get to go out to national parks and go to hot springs. I take bits of the microbial mat itself – a living community of multiple microbes – and bring it back to the lab and study the lipid biomarkers in that.
Part of your research involves looking at how these microbes form or operate under very extreme conditions. What have you learned so far about how life forms in those environments?
Welander: These microbes have really learned how to modify their external structure, their membranes, using these lipids to help them withstand extreme conditions. They can use these molecules to make their membranes more rigid if an environment is too hot or more flexible if the environment is too cold. It shows that these molecules serve a purpose in helping them withstand these environmental stressors that they may encounter in their everyday environments.
Why did you start studying microbial lipid biomarkers? And what do you like about it?
Mayer: I took a class my second year of college that was called Biodiversity. We learned about every different branch of life and how it interacts with the Earth that we live on and that totally took my breath away. That was the first time I heard about microbes driving everything about the planet, from today to back when life first evolved, and I really wanted to find a way to study their fossils.
What is the most surprising thing that you’ve encountered in your time studying microbial lipid biomarkers?
Welander: You can have something that’s made by a bacterium and something that’s made by an alga, which are two different types of microbes from a geological and biological perspective. But then these two organisms make the same molecule using completely different genes and completely different proteins. That’s really an exciting concept and shows that this must be an important molecule for these organisms if they evolved independent mechanisms to make it.
Mayer: I think for me the most surprising aspect is how totally ubiquitous they are across all life. Every form of life has some kind of lipid in its membrane that is functionally or structurally similar to the lipid biomarkers that I’ve been studying in the lab during my PhD.
Let’s talk a little bit about astrobiology. How do you see your research relating to the search for life on other planets?
Welander: The core of astrobiology is that we are trying to understand whether there is life out there in the universe besides our own. One of the ways that our work is relevant is it’s showing us what is life capable of making that can be preserved – could we find these molecules on different planets?
Mayer: If there was life on Mars, you can perhaps use the same lessons we’ve learned about the early Earth to study Mars. That’s where I hopefully see my research going one day.
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